Earlier quoted context omitted.
Those "who want to learn it from them" must make the assessment. Any less is censorship of the teacher, which, in my mind, is an unqualified evil. This is not to say that the material learnt/shared is an unqualified good. The ability of each individual to freely exchange that knowledge and assess it in their context is.
But then how would you separate, for example, teaching from preaching?
Totally fine to restrict subjects that can be taught in schools though, including religion and the like.